The present invention relates to the recovery of multiphase modulated digital data, and more particularly to a method and apparatus for recovering high quality digital audio signals transmitted via a cable television system or over the airwaves.
New digital techniques for the reproduction of sound provide performance that is far superior to analog techniques which have been used in the past. An example of high fidelity sound reproduction using digital techniques can be found in the compact disc technology that has enjoyed tremendous success as an alternative to phonograph records and tapes. Digital recording and playback techniques provide reproduction of music that is extremely realistic and absent from background noise and distortions which have plagued other high fidelity sound reproduction systems in wide scale use.
Cable television ("CATV") networks can provide one medium for the transmission of digital audio programs. Cable television growth has come from the development of various programming categories and by the technologies which have made program delivery possible. Cable first brought distant television signals to areas where there was little or no off-air reception. This applies to distant signals and weak signal areas where outdoor antennas were previously mandatory. The next category to bring major growth to cable was pay service after the development of reasonable cost satellite delivery systems. After satellite delivery became less costly and was accepted in the industry, super stations and cable networks formed another category of programming that has become customary, providing "extended basic" services. Franchising and local politics have created a generally unprofitable but necessary category called "local origination". Recently, addressable technology and aggressive marketing have enabled "pay-per-view" programming to proliferate.
The provision of high quality audio services over cable is rapidly becoming available. In the past, the signal quality of analog audio transmission techniques has been poor and there has been no efficient way to collect revenue or control access to such services.
Digital techniques for the communication of high fidelity audio programs via a CATV network as well as via direct broadcast satellite and over the air enable the provision of high quality audio services on a subscription basis. Commonly owned U.S. Pat. application Ser. No. 07/280,770 filed Dec. 6, 1988, and now U.S. Pat. No. 5,038,402, for "Apparatus and Method for Providing Digital Audio in the FM Broadcast Band", incorporated herein by reference, discloses a method and apparatus for providing high quality digital sound signals within the FM broadcast band. Commonly owned U.S. Pat. No. 4,821,097 to C. Robbins entitled "Apparatus and Method for Providing Digital Audio on the Sound Carrier of a Standard Television Signal", also incorporated herein by reference, discloses a system wherein the FM audio portion of a standard television signal is replaced with digital audio. In the techniques disclosed in these references, the digital audio information is carried using multilevel (e.g., multiphase) modulation.
A well known type of multiphase modulation is quadrature phase shift keyed ("QPSK") modulation. Advantages of QPSK modulation are discussed in C. Robbins, "Digital Audio for Cable Television", 1986 NCTA Technical Papers, Dallas, Texas, Mar. 15-18, 1986, pp. 21-24.
In a QPSK transmission, an in-phase component I and an out-of-phase component Q (typically, 90.degree. out of phase with the I component) are provided. The I and Q components are processed in a well known manner to recover the original digital data that was used to modulate the QPSK signal. One known technique for detecting the I and Q components from the transmitted signal on which they are carried uses an independent phase reference derived independently of the modulated data carrier. An example of such a demodulator is provided in FIG. 6 of the above-mentioned U.S. Pat. No. 4,821,097. Various analog and analog/digital schemes that do not require an independent phase reference are also known.
It would be advantageous to provide demodulation of multiphase modulated data using purely digital techniques. This would allow the demodulator to be constructed from a very large scale integrated circuit ("VLSI") at a cost advantage over prior art systems. It would be further advantageous to provide such a digital demodulator that does not require an independent phase reference to be carried in the transmitted signal, and which can be implemented using a digital delay without the need for a complicated digital to analog converter. It would also be advantageous for such a demodulator to include an automatic frequency control ("AFC") to enable accurate recovery of the modulated phase components using the data carrier itself as a reference.
The present invention provides a method and apparatus enjoying the above-mentioned advantages.